My sister and I caught a little fish in Sawyers Bay Creek. It was
unusual in that it had a sac on its tail. Can you tell us about the fish
and the sac? P.S. We put the fish back.

Gerry Closs, a freshwater ecologist at the University of Otago, responded.

The small fish in the photograph is a banded kokopu, one of four native species of
galaxiid that are found in coastal streams around New Zealand. The sac around its
tail is a fungal infection caused by the water mould Saprolegnia, a common disease
of this species in autumn and winter. Spawning and low winter temperatures often
stress fish, and when stressed, fish may develop Saprolegnia infections that can kill
if not treated. Saprolegnia is found in most freshwater systems, and can also be a
problem in overcrowded or poorly maintained freshwater aquaria. The coat of slime
that surrounds a fish’s body is the primary defense against fungal infections, and
is easily damaged (that’s why you should handle live fish with wet hands). If the
slime coat is damaged, fungal infections can establish, and then spread over the
fish’s body. Unlike most fish, galaxiids have no scales, and this may make them
particularly susceptible to fungal infections.

Can fungal infections be treated? If caught early and only cover a small area of a
fish’s body, then effective commercial preparations are available from aquarium shops.
Saltwater baths for 24 hours also work – a teaspoon of salt for each litre of water
will treat most fish successfully. However, fish will remain vulnerable to new infections
unless the stress that caused the infection is removed. For the banded kokopu in the
picture, such a treatment might have worked, although this is by no means certain as
the infection was well advanced. Further, the stress that led to the infection is part
of the natural annual cycle, and once returned to the wild, the risk of the disease
redeveloping would be high. Sadly, given the extent of the infection, it probably
died within a day or two of being returned to the stream.